1. Field of the Invention
This invention relates to an image processing method for forming an image suitable for image diagnosis such as abnormal shadow detection, an apparatus for carrying out the image processing method, a computer program for causing a computer to execute the image processing method, a computer readable medium on which the program is recorded, an abnormal shadow detecting method using the image processing method, a computer program for causing a computer to execute the abnormal shadow detecting method, and a computer readable medium on which the program is recorded.
2. Description of the Related Art
When detecting an abnormal shadow representing a growth or the like on the basis of a medical image representing a predetermined affected part, so-called energy subtraction has been carried out in order to make invisible the bone which is an obstruction to the detection.
Energy subtraction is a process for generating an image free from a part of the structure element of an object by preparing two images of the same object different in X-ray dose and taking the difference between the two images on the basis of the fact that the X-ray absorption for a given amount of X-rays irradiated to the object differs from structure element to structure element of the object. That is, by subtracting a bone-enhanced image (an image in which bones are enhanced) from a soft-part-enhanced image (an image in which soft parts are enhanced), an image free from bones can be obtained. An image obtained in this way is referred to as “an energy-subtraction image”. See U.S. Pat. No. 4,855,598 (will be referred to as “patent publication 1”, hereinbelow).
Further, when detecting an abnormal shadow representing a newly generated growth or the like, so-called temporal subtraction has been carried out in order to obtain an image of a predetermined affected part in which the abnormal shadow is enhanced.
Temporal subtraction is a process for generating an image in which change with time of the affected part is enhanced by preparing two images of the same object taken in different times and taking the difference between the two images. When an image representing the part before generation of the disease and normal is used as a past image, an image in which an abnormal shadow representing a newly generated growth or the like is enhanced can be obtained. See U.S. Patent Laid-Open No. 20010002934 (will be referred to as “patent publication 2”, hereinbelow).
However, there has been a problem in energy subtraction in that two images of the same object different in X-ray dose must be prepared, and there has been a problem in temporal subtraction in that two images taken in different times must be prepared. That is, the two processes both require a plurality of images, and an image from which bones are removed or in which an abnormal shadow is enhanced cannot be obtained only a single image taken in a simple manner.
In order to obtain an image from which bones are removed or in which an abnormal shadow is enhanced, a special system and/or a special technique is required, which results in the fact that an image necessary to the image diagnosis cannot be effectively generated.
Further, in the medical field, there has been put into practice computer-aided abnormal shadow detection (CAD) in which an abnormal shadow in a digital medical image is automatically detected by the use of a computer. As the computer-aided abnormal shadow detection (CAD), there has been known chest CAD in which, on the basis of a digital chest X-ray image, a growth shadow in the chest is detected.
The chest X-ray image includes so-called feature background image comprising images of structure elements having various anatomical features such as ribs, a clavicle and the like, which is an obstruction to detection of an abnormal shadow and deteriorates the detection capability.
U.S. Pat. No. 5,289,374 (will be referred to as “patent publication 3, hereinbelow) proposes a technology in which chest CAD is performed after removing the feature background image by filtering.
However, since a chest is complicated in anatomical structure and the feature background image cannot be satisfactorily removed from the chest X-ray image by the chest CAD employing filtering, the detection capability cannot be improved by the approach.